Method of producing ohmic contacts on semiconductors



Feb. 15, 1966 D. R. BOYD ETAL 3,235,476

METHOD OF PRODUCING OHMIG CONTACTS ON SEMIGONDUCTORS Original FiledApril 18. 1960 United States Patent 3 235,476 METHOD OF PRODCING HMICCONTACTS ON SEMICONDUCTORS David R. Boyd, Royal Oak, Yro T. Sihvonen,Birmingham, and Calvin D. Woelke, Detroit, Mich., assignors to GeneralMotors Corporation, Detroit, Mich., a corporation of Delaware Originalapplication Apr. 18, 1960, Ser. No. 23,038, now Patent No. 3,121,852,dated Feb. 18, 1964. Divided and this application Feb. 21, 1963, Ser.No. 266,124 4 Claims. (Cl. 204-192) This application is a division ofour copending United States patent application Serial No. 23,038, Boydet al., now Patent No. 3,121,852, entitled Ohmic Contacts onSemiconductors, which was filed April 18, 1960.

This invention relates to semiconductor devices. More particularly theinvention pertains to transparent ohmic contacts on semiconductordevices, as well as to the method and apparatus by which such contactsare formed.

Certain semiconductors, such as cadmium sulfide, exhibitphoto-conducting properties which can be utilized for a variety ofpurposes. However, use of such a semiconductor heretofore has beenrestricted because it was not possible to realize the fullest potentialof the Iadvantages obtainable therewith. A most important factor whichimpeded the full realization of these benefits was the impossibility ofmaking a suitable, low resistance, transparent ohmic contact with thesemiconductor crystal.

It is well recognized that the resistance of an electrical contact isdecreased by increasing the contact area. IFor this reason lowresistance electrical connections generally involve initially attachingan electrical `contact to a comparatively large surface area on thesemiconductor and thereafter soldering an electrical lead to thecontact. The electrical contact yattached to the semiconductor can be arectifying Contact or an ohmic contact. It is toward this latter type ofcontact that our invention is directed.

Suitable low resistance ohmic contacts on semiconductors must not onlyinvolve the largest feasible contact area but the contact must have anintimate association with the semiconductor. Although various means canbe used to attach an electrical contact to the surface of asemiconductor not all of these means provideV the intimate associationbetween contact and semiconductor that is [required for lowestelectrical impedance. The most `suitable ohmic contacts 'are opaque andimpenetrable by electromagnetic means such as visible and ultravioletlight. However, a comparatively large lopaque contact on the surface ofthe crystal simultaneously inhibits irradiation of the semiconductor.Thus, to utilize the photoconducting properties of such a semiconductorit was heretofore necessary to sacrifice lowest possible resistance byusing comparatively small contacts. A compromise between lowestimpedence and maximum generation of photocurrents had to be made.

Our invention eliminates this compromise by providing a conductivetransparent film which can be intimately secured to Ia semiconductor.Our invention can be used to make a semiconductor ohmic contact whichpermits maximum generation of photocurrents with minimum impedence tothe flow of electrons therethrough. Our invention provides both a methodyand apparatus for making tran-sparent ohmic contacts on suchsemiconductors.

Other objects, features and advantages of this invention will becomemore apparent from the following descrip- ICC tion `of preferredembodiments thereof and from the drawing, in which:

FIGURE l is a schematic view showing an apparatus contemplated by theinvention as useful in forming transparent ohmic contacts on asemiconductor;

FIGURE 2 is a diagrammatic view showing a semiconductor which is formedin accordance with our invention and which is connected in an electricalcircuit in such a manner that the intensity of light impinging thereonregulates the ow of electrons through the electrical circuit; and

FIGIURE 3 is another diagrammatic view showing a modification of theinvention shown in FIGURE 2.

Our invention comprehends sputtering a tin-indium alloy onto the surfaceof the `semiconductor in an oxygen atmosphere to form a transparentelectrically conductive lm thereon which functions as an ohmic contact.The method by which the transparent coating is applied to thesemiconductor can more expeditiously be described in connection with theapparatus used. For this reason a prior description of the .apparatuswould be fruitful land reference is herewith made to FIGURE 1.

The `apparatus in FIGURE l has a closed chamber 10 which is `formed by ametal base plate 12, a glass housing 14 and an upper electrode support16. Resilient seal members 18 and 20, respectively, are disposed betweenthe base plate and housing and between the housing and upper electrodesupport. The seals 18 and 2t) form imperforate junctions between thevarious described members cooperating therewith to permit evacuation ofthe chamber.

A copper electrode 22 having a coating 24 thereon of a tin-indium alloyis secured to the bottom of the upper electrode support 16 which dependsinto the chamber 10. The electrode 22 is in electrical -contact with thesupport 16 but is removable therefrom to facilitate coating thereof. Theelectrode 22 can be attached to the support 16 by means of a stud 26which is in threaded engagement with a recess in the lower end of thesupport 16.

A second electrode 28 within the chamber 10 is disposed on and inelectrical communication with the base plate 12. The base plate can beof any suitable metal, such as aluminum, and the electrode 28 `can be ofaluminum, As this electrode need not be removable, the seal between itand the base plate can be accomplished in any conventional manner as bysoldering. The upper end of the electrode 28 is 'substantiallyhorizontal forming a table 30 on which llies a substrate 32 to becoated. In further reference to the electrode 28 it will be designatedIas the tab-le electrode to distinguish it from the upper electrode 22.A glass plate 36 is preferably used to space the substrate 32 from thetable electrode 28 to restrict any interaction therebetween.

As operation of the apparatus involves a heating of the variouselectrodes and parts associ-ated therewith, provision is made to lcoolthese parts. The table electrode 28 is hollow to permit a coolant to becirculated therewithin to not only cool the electrode but also thesub-strate or crystal 32 and glass plate 36 lying thereon. A portion 38of the table electrode projects downwardly through an aperture in thebase plate 12 forming an outlet 40 for a liquid coolant which isintroduced into the electrode through the tube 42. The upper electrodesupport has a cooling chamber 44 therein through which a liquid coolantis circulated. The coolant is introduced into the support via the tube46 and exits the cooling chamber via the outlet tube 48.

A direct current power supply 50, which is reversible in polarity, isconnected to the base plate through the electrical lead 52 and to theupper electrode support through the electrical lead 54.

Evacuation of the chamber is accomplished by a vacuum pump (not shown)which communicates with the chamber by means of a tube 56 and aperture58 in the base plate 12. A water trap 60 is provided in the vacuum line56 between the vacuum pump (not shown) and the chamber to removemoisture from the system.

Means for introducing a selected gas into the chamber is providedthrough another aperture 62 in the base plate. The yselected gas, sucha-s oxygen, can be obtained from a bottle of compressed oxygen gas 64.Accurate control of the introduction of oxygen into the chamber can `beobtained through a bleed valve 66. Utilization of a pressure monitor 68can vadditionally permit extremely accurate regulation of pressure inthe chamber while bleeding in oxygen gas.

A description of the manner in which the apparatus shown in FIGURE 1 isused is also intended to serve as a description of the method of ourinvention. Before treatment of a semiconductor crystal 32 in theapparatus described above, it may be desirable to perform preliminaryoperations thereon. In such instance the particular preliminaryoperations which are to be conducted on the crystal will be dependentupon the nature of the final product being made. These preliminaryoperations may be material in enhancing the characteristics of aspecific product but operability and utility of our invention are notdependent thereon.

By way of example a cadmium sulde crystal can be cut into the desiredconfiguration in the manner known and accepted in the art. As thecutting or slicing operation frequently involves sawing with a diamondor carbide tipped saw, it may be desirable to lightly lap the surface ofthe crystal slice to remove saw marks. The lapping can be performed with#600 silicon carbide or silicon boride grit. After the lapping operationthe crystal is rinsed in a suitable solvent, such as acetone, dried andplaced on the glass plate 36 on the table 30 of the table electrode. Itis understood, of course, that other preliminary treatments can be usedin addition to or in place of those described above.

The housing 14 and housing supported members 16, 18 and 22 are thenplaced over the base plate 12 and evacuation of the chamber 10 iscommenced. Concurrently circulation of the liquid coolants through theelectrode support 16 and the table electrode 28 can be commenced.

The chamber 10 is preferably evacuated by the vacuum pump to a pressurebelow about 100 microns of mercury. Oxygen is then bled into the chamberuntil the pressure is raised to almost atmospheric pressure. The chamberis then evacuated once again to a pressure below 100 microns of mercuryand oxygen bled into the chamber until the desired pressure obtains. Inthis manner the chamber is purged of contaminating gases and asubstantially pure oxygen atmosphere can be obtained. The chamber can berepeatedly purged in this manner to obtain an even purer oxygenatmosphere. The number of purgings that may be desired, of course,depends upon the pressure to which the chamber is evacuated before theoxygen is introduced. The lower the evacuation pressure the greater theeffectiveness of the purging. When the charnber is evacuated to apressure of below about 10 microns of mercury before the oxygen is bledin, only one purging may be required.

After the chamber has been purged the pressure is adjusted toapproximately 100 microns of mercury and a negative potential ofapproximately 2000 volts to 2500 volts is applied to the table electrode28. Under these conditions a reverse sputtering of the semiconductor iseffected. The potential is maintained for at least two minutes whereuponit is reduced to zero.

The oxygen pressure is then increased to approximately 150 microns bybleeding oxygen into the chamber and then the addition of oxygen isceased. The polarity of the power supply is then reversed into thenormal sputtering arrangement in which the upper electrode 22 forms thecathode. The potential is gradually increased to about 1500 volts whilethe pressure is concurrently being reduced. After the voltage hasreached approximately 1500 volts, oxygen is again bled into the systemand the voltage gradually increased to about 2000 volts to 2500 volts.The rate at which voltage is increased is preferably taken inassociation with changing pressure so as to maintain a current flow ofabout 30 milliamperes to 40 milliamperes at all times.

Once the potential of approximately 2000 volts to 2500 volts has beenattained the oxygen pressure can also be adjusted, if required, tomaintain a constant current of approximately 30 milliamperes to 40milliamperes. The oxygen pressure generally found necessary to obtainthis current flow is about microns of mercury to 80 microns of mercury.The system is retained at this voltage and pressure for approximatelyminutes. Under these conditions the material of the cathode coating 24,the tin-indium alloy, is sputtcred into the oxygen atmosphere causing adeposition of a transparent electrically conductive film on thesemiconductor surface.

After a film of sufficient thickness has been achieved, the voltage isreduced to zero. Although the lm resulting in the above deposition istransparent and has a satisfactory conductivity, its conductivity can beincreased even further if it is subjected to the following posttreatment.

After the voltage is reduced to zero, as indicated above, the pressureis increased to approximately microns of mercury, again by bleeding inoxygen. At about 150 microns of mercury pressure the polarity of thepower supply is reversed and a negative potential of approximately 1500volts is applied to the table electrode. This potential is maintainedfor approximately 60 seconds at which time the potential is reduced tozero. The pressure is thereafter increased to atmospheric, the crystalremoved from the chamber and cleaned with any of the known solvents,such as toluene and then acetone.

Although the transparent coating can be formed equally well if thesemiconductor is placed directly on the table, we prefer to interposethe glass plate therebetween. It has been found that the crystal mayexhibit an interaction with the table electrode deleteriously affectingthe surface of the crystal in contact therewith. Effective insulationfrom this interaction has been achieved using a glass plate slightlylarger in surface area than the crystal.

As the sputtering treatment causes a temperature increase of thesemiconductor crystal it is especially important to provide effectivemeans for removing heat generated therein. Thus, insulating means mustnot only restrict interaction between the crystal and the tableelectrode but also function as a means for conducting heat away from thecrystal to the water cooled table electrode. Glass has been found to beadequate for both of these purposes. However, in some instances, it maybe preferred to apply quartz, recrystallized alumina or mullite.

The faster the rate of sputtering, the higher the temperature to whichthe semiconductor is raised. The more eicient the cooling of thesemiconductor, the lower its temperature for a given rate of sputtering.Thus, more ecient cooling permits one to employ a faster rate ofsputtering. Cooling is more efcient if the contact between the tableelectrode and parts thereon is intimate. To attain a more intimatecontact layers 70 and 72 of silicone grease are, respectively, placedbetween the semiconductor and the glass plate and between the glassplate and the electrode table. We generally prefer to apply the siliconegrease to both of two contacting surfaces to insure continuity of thefilm of grease therebetween. A more effective cooling is thus obtained.

lt is a further function of -the grease to hold the various componentson the table electrode in assembly and it is also believed that thegrease additionally inhibits a secondary sputtering between the glasssurface and the semiconductor surface which is in contact therewith. Anyinert material that has a low vapor pressure and which is sufficientlystable to withstand the sputtering treatment, such as iiuorocarbongreases and waxes, might be used in place of the silicone grease.

Although we prefer to clean the semiconductor surface by means of areverse sputtering treatment befort the transparent ohmic contact isapplied, in some instances it may be preferred to chemically etch thesemiconductor surface in the normal and accepted manner for suchetchings. In such instance, when etching a cadmium sulfide crystal,etching for two minutes in concentrated hydrochloric acid orconcentrated nitric acid can be used. Although chemical etchants may besatisfactory for some purposes, it is generally preferred to use thereverse sputtering treatment to clean the semiconductor surfaceirnmediately prior to the application of the sputtered transparentcoating hereon. Reverse sputtering will effectively clean the surfacewithout presenting the problem of possible concurrent contaminationthereof.

The position of the semiconductor in the apparatus is no more materialto our invention than it is to usual sputtering practices. By this wemean that a sputtered coating can be formed by locating thesemiconductor within the chamber oher than on the table electrode.Although a sputtered coating might be obtained with a semiconductor atanother location, thicker coatings are obtained at a faster rate andgenerally of superior quality when the semiconductor is placed in adirect line between the negative and positive electrodes. Coating metalwhich is released from the cathode has a greater tendency to be directedtoward the anode. Thus, substances placed interjacent the electrodeswould come into contact with a greater proportion of the coating metalreleased from the cathode than in any other location.

The electro-des are spaced in the customary manner and the semiconductoris preferably placed in a line between the electrode closely adjacentthe table electrode. This electrode is the positive electrode forsputtering the tinindium alloy onto the semiconductor. In this mannernot only is the semiconductor most susceptible to coming into contactwith the greatest proportion of the coating metal but also issufficiently far enough away from the cathode to have the coatinguniformly and coextensively distributed throughout the exposed surfaceof the semiconductor.

The voltage which is applied during sputtering, in general, must besufficiently high to obtain sputtering at a satisfactory rate. However,when too high a voltage is employed there may be a deleteriousoverheating of the sub-strate when a sputtered coating is being appliedto a semiconductor. Thus, the upper limit of potential when coating aheat sensitive substrate is that at which deleterious overheating of thesubstrate occurs. On the other hand, if the substrate which is beingcoated is not deletriously affected by such temperature increases, theupper limit of potential during sputtering is that at which sparkingwould occur between the two electrodes. The reverse sputtering as wellas the sputtering to form the transparent coating can be satisfactorilyaccomplished at a potential of about 2000 volts to 2500 volts when thesubstrate is a cadmium sulfide crystal. Similarly, the duration of thesputtering will be dependent upon the rate at which the variousmaterials will sputter. The reverse sputtering to clean the crystal needonly be about two minutes for cadmium sulfide, cadmium selenide orcadmium telluride. Reverse sputtering to clean semiconductors formed ofany of the Gro-up II metals will be generally satisfactory for mostpurposes if of an equal duration.

The pressure at which the sputtering or reverse sputtering isaccomplished is about 50 microns of mercury to 200 microns of mercury.Although a lower pressure can Abe used, unreasonable lengths of time forcleaning become involved, while a pressure higher than about 200 micronsof mercury may entirely prevent the sputtering process from occurring.The preferred pressure used is yprimarily dependent upon the voltageapplied.

Heating of a cadmium sulfide crystal above a temperature of about 400 C.induces disassociation and sublimation of sulfur present therein leavingpure cadmium on the surface of the crystal. Such action affects thephotoconducting and luminescing properties of the crystal. With theliberation of free cadmium in the crystal lattice, oxygen can diffusetherein changing the stoichiometry of the crystal decreasingluminescence but increasing sensitivity and absorpt-ion in the nearinfrared.

Before the transparent coating is sputtered onto the crystal it isdesired to firs-t reduce the oxygen pressure so as to eliminateoutgassing during the sputtering step. For this reason we prefer toreduce the pressure within the chamber to below 100 microns andconcurrently increase the negative potential on the upper electrode toabout 1500 volts. At this point there is little sputtering butoutgassing in the upper regions of the sputtering chamber and upperelectrode occurs. During the outgassing -there are sporadic increases inpressure and violent surges in deposition rate. The Voltage ismaintained at approximately 1500 volts until outgassing subsides. Therate at Which pressure is decreased and voltage -is increased ispreferably predetermined to maintain a current 4of approximately l30milliamperes to 40 milliamperes.

After outgassing has subsided the system is ready to produce a moresatisfactory transparent sputtered coating. At this point the potenti-alis raised to approximately 2000 volts to 2500 volts and oxygen pressureis concurrently increased. The rate of potential and pressure increaseis so regulated as to maintain the .current a-t approximately 30milliamperes to 40 milliam-peres. The precise duration of the sputteringtreatment depends upon the thickness of the coating which is desired. Ingeneral a duration of approximately minutes provides a satisfactorycoating thickness.

The precise nature of our coating is somewhat uncertain but it appearsto be a reaction product of the indium-tin alloy with the oxygen gas.X-ray and spectrochemical analyses indicate that the film is a mixtureof In203 with tin. This is supported by the observation that oxygenpressure decreases during formation of the -coating if no oxygen isadded to the system. Accordingly, it is generally desirable to*concurrently bleed oxygen into the system during the sputtering processto replace the oxygen atoms which are utilized in forming the film.

The film resulting 'from the oxygen sputtering of the tin-indium alloypossesses excellent transparency and a highly satisfactory degree ofconductivity. By oxygen sputtering of an alloy we refer to a sputteringprocess, as described herein, in which the alloy forming the activesurface of the negative electrode is sputtered in an oxygen atmosphere.The mate-rial deposited in the process is a reaction product of thesputtered alloy and the oxygen. The film resulting in the oxygensputtering is a highly satis-factory contact on a semiconductor.However, it has been found that the conductivity of the film `can evenbe increased if it is subjected to reverse sputtering treatment forabout a minute. It is not certain how the conductance of the film ismaterially improved iby the reverse sputtering treatment but may be aresult of additional surface heating and/or oxidation. The reversesputtering treatment, of course, should be very short compared to theduration of film deposition as reverse sputtering tends to remove filmmaterial. A substantial improvement in conductivity has been achievedwhen a film which has been deposited for 75 minutes is reverse sputteredfor only one minute.

It is essential to attaining of a transparent `film that the coatingused on the upper electrode be of a particular composition. Especiallysatisfactory results have been obtained using a tin-indium alloy coatinghaving a tin content of approximately 18%. However, satisfactory resultshave been obtained with tin-indium alloys having from about to 70% tin.An extremely important characteristic of the tin-indium alloy is that itdoes not combine with the semiconductor to alter the conductivity typethereof but rather forms an intimate ohmic contact therewith. Indium andtin do not adversely affect the conductivity type of an n-typesemiconductor such as cadmium sulfide, cadmium selenide and cadmiumtelluride and, therefore, are extremely advantageous `for making anohmic contact thereon. Semiconductors made from the elements of Group IIof the Periodic Table of Elements may also be similarly coated.

It may be desired to also form a `second transparent ohmic contact onthe crystal. This contact can be formed on the surface 74 of the crystalwhich was in contact with the glass plate in the previously describedmethod. In such instance the crystal is first coated as described above,cleaned and then reinserted in the chamber in an inverted position. Thesecond transparent coating would then be applied in the same manner asthe first.

The resultant article would then have a transparent conductive coatingon opposite surfaces of the crystal and can be used in an electricalcircuit such as shown in FIGURE 2. Referring now to FIGURE 2, thecrystal 32 has electrical leads 76 and 78 from a power source 80respectively attached to each of these contacts inducing an electricalpotential therebetween. The amount of current 4passing between the twocontacts can be regulated by the intensity of light, I0, impinging onthe crystal. A plurality of such devices can be arranged adjacent oneanother so that radiation successively passes through one suchsemiconductor into the other. In this manner more efficient use ofradiation of a given intensity is obtained.

In some instances it may be desired to use a reflective contact incombination with a transparent contact. The transparent contact, ofcourse, can be applied in the manner hereinbefore described. Therefiective coating would be formed on the surface 74 of the crystalopposite to that having a transparent coating thereon. The manner inwhich the reflective coating is applied forms no part of this inventionand may be accomplished in any suitable manner. For example, thereflective coating can be formed by electrodeposition in the mannerdescribed in the copending United States patent application Serial No.677,914, Boyd et al., filed August 13, 1957, and which is owned by theassignee of the present invention. The reiiective `coating may also beapplied by evaporation techniques which are well known in the art. Theresultant article is shown in FIGURE 3 where electrical leads 82 and 84from a power source 86 are respectively connected to the transparentcoating and the refiective coating inducing an electrical potentialtherebetween. The amount of current flow through the semiconductor 32"can then be regulated by the intensity of radiation impinging on thesemiconductor. In this embodiment of the invention the amount of currentfiow is substantially increased over that obtained with the embodimentshown in FIGURE 2 since light, I0,` impinging on the crystal through thetransparent coating passes through the crystal, strikes the refiectivecontact and is reflected back through the crystal. Thus, a double effectis obtained.

Although the invention has been described in connection with certainspecific examples thereof, no limitation is intended thereby except asdefined in the appended claims.

We claim:

1. A method for the production of thin films which comprises thefollowing steps: placing a substrate in a closed chamber, providing alow pressure oxygen atmosphere within said chamber, providing in saidchamber a first electrode having an active surface thereon which isformed of a tin-indium alloy containing about 10% to 70%, by weight, tinand the balance substantially indium, providing a second electrodewithin said charnber, applying a negative potential to said secondelectrode so as to clean said substrate by reverse sputtering, andapplying a negative potential to said first electrode, said potential onsaid rst electrode being sufficient in degree and duration at saidpressure to sputter said tinindium alloy into said oxygen atmospherecausing a deposition of a transparent electrically conductive film onsaid substrate material.

2. A method for the production of thin films which comprises thefollowing steps: placing a substrate in a closed chamber, providing alow pressure oxygen atmosphere within said chamber, providing in saidchamber a first electrode having an active surface thereon which isformed of a tin-indium alloy, containing about 10% to 70%, by weight,tin and the balance substantially indium, providing a second electrodewithin said chamber, applying a negative potential to said secondelectrode so as to clean said substrate by reverse sputtering, applyinga negative potential to said first electrode, said potential on saidfirst electrode being sufiicient in degree and duration at said pressuret0 sputter said tin-indium alloy into said oxygen atmosphere causing adeposition of a transparent electrically conductive film on saidsubstrate material, reapplying said negative potential to said secondelectrode for a substantially lesser duration than that at which saidfilm is deposited so as to improve electrical conductivity of said filmand cooling said substrate during application of said negativepotentials.

3. The method for the production of thin films which -comprises thefollowing steps: providing in a closed chamber a first electrode havingan active surface which is formed of a tin-indium alloy containing about10% to 70%, by weight, tin and the balance substantially indium,providing a second electrode in said chamber, placing a radiationsensitive semiconductor selected from the group consisting of cadmiumsulfide, cadmium selenide and cadmium telluride in said chamber adjacentsaid second electrode, providing a low pressure oxygen atmosphere withinsaid chamber, applying a negative potential to said second electrode soas to clean said semiconductor by reverse sputtering, at an oxygenpressure of approximately 50 millimeters of mercury to 200 millimetersof mercury applying to said first electrode a negative potential ofapproximately 2000 volts to 2500 volts, said potential being of asufficient duration to sputter said tin-indium alloy into said oxygenatmosphere causing a deposition of a transparent electrically conductivefilm on said semiductor, reapplying said negative potential to saidsecond electrode at a low oxygen pressure for a substantially lesserduration than that at which said film is deposited so as to improveelectrical conductivity of said film and cooling said substrate duringapplication of said negative potentials.

4. A method for the production of thin films which comprises thefollowing steps: in a closed chamber providing a first electrode havingan active surface which is formed of a tin-indium alloy containing about10% to 70%, by weight, tin and the balance substantially indium,providing a second electrode, placing a radiation sensitivesemiconductor from the group consisting of cadmium sulfide, cadmiumselenide and cadmium telluride in said chamber adjacent said secondelectrode, applying a negative potential to said second electrode so asto clean said semiconductor by reverse sputtering, providing within saidchamber an oxygen atmosphere at a pressure of about 50 millimeters ofmercury to 200 millimeters of mercury, and applying a negative potentialof about 2000 volts to 2500 volts to said first electrode so as todeposit a transparent conductive film on said semiconductor.

References Cited by the Examiner UNITED STATES PATENTS 2,465,713 3/1949Dimmick 117-933 2,636,855 4/1953 Schwarz 204-192 (Other references onfollowing page) 9 10 UNITED STATES PATENTS OTHER REFERENCES 2,766,14410/ 1956 Lidow 338-15 Holland: Vacuum Deposition of Thin Flins, pp. 491-2,769,778 11/1956 Preston 2041 92 498, 1956. 2,937,353 5/1960 Wasserman338--15 3,021,271 2/ 1963 Wehner 204-192 5 WINSTON A. DOUGLAS, PrimaryExaminer.

FOREIGN PATENTS JOHN H. MACK, Examiner.

542,599 1/ 1942 Great Britain. v830,392 3/1960 Great Britain.

1. A METHOD FOR THE PRODUCTION OF THIN FILMS WHICH COMPRISES THEFOLLOWING STEPS: PLACING A SUBSTRATE IN A CLOSED CHAMBER, PROVIDING ALOW PRESSURE OXYGEN ATMOSPHERE WITHIN SAID CHAMBER, PROVIDING IN SAIDCHAMBER A FIRST ELECTRODE HAVING AN ACTIVE SURFACE THEREON WHICH ISFORMED OF A TIN-INDIUM ALLOY CONTAINING ABOUT 10% TO 70%, BY WEIGHT, TINAND THE BALANCE SUBSTANTIALLY INDIUM, PROVIDING A SECOND ELECTRODEWITHIN SAID CHAMBER, APPLYING A NEGATIVE POTENTIAL TO SAID SECONDELECTRODE SO AS TO CLEAN SAID SUBSTRATE BY REVERSE SPUTTERING, ANDAPPLYING A NEGATIVE POTENTIAL TO SAID FIRST ELECTRODE, SAID POTENTIAL ONSAID FIRST ELECTRODE BEING SUFFICIENT IN DEGREE AND DURATION AT SAIDPRESSURE TO SPUTTER SAID TININDIUM ALLOY INTO SAID OXYGEN ATMOSPHERECAUSING A DEPOSITION OF A TRANSPARENT ELECTRICALLY CONDUCTIVE FILM ONSAID SUBSTRATE MATERIAL.